The present invention is directed to a main rotor blade for rotary-wing aircraft and more particularly to a main rotor blade twist distribution for a rigid coaxial, counter-rotating rotary-wing aircraft capable of hovering and cruising at speeds in excess of 250 kts.
Conventional rotary-wing aircraft have a forward flight speed limited by a number of factors. Among these is the tendency of the retreating blade to stall at high forward flight speeds. As the forward airspeed increases, the airflow velocity across the retreating blade slows such that the blade may approach a stall condition. In contrast, the airflow velocity across the advancing blade increases with increasing forward speed. Forward movement of the aircraft thereby generates an asymmetry of lift between the advancing and retreating sides of the rotor. This asymmetry of lift may create an unstable condition if not equalized across the advancing and retreating sides of the rotor.
A rotary wing aircraft with a coaxial (or other) counter-rotating rigid rotor system is capable of higher speeds compared to single rotor helicopters due in part to the balance of lift between the advancing sides of the main rotor blades on the upper and lower rotor systems. In addition, the retreating sides of the rotors are also generally free from classic retreating blade stall that conventional single or dual rotor helicopters may suffer from because they are not required to produce lift.
To still further increase airspeed, such a rotary wing aircraft may incorporate an auxiliary translational propulsion system. Use of a rigid coaxial counter-rotating rotor system in combination with an auxiliary translational propulsion system, allows such a rotary-wing aircraft to attain significantly greater speeds than conventional rotary-wing aircraft, while maintaining hover and low speed capabilities.
One system significant to these flight attributes is the design of the main rotor system of which the rotor blades are the primary force and moment generating components. Design requirements for a rotary-wing aircraft incorporating a rigid counter-rotating rotor system differ significantly from conventional rotary-wing aircraft. As with a conventional rotary-wing aircraft, the advancing blades of both the upper and lower rotors produce lift; however, unlike a conventional single or multi-rotor rotary-wing aircraft, the retreating blades of the counter-rotating rotor are off-loaded commensurate with increasing flight velocity, and need not produce lift to balance lateral (rolling) moments. Rather, roll equilibrium is attained by balancing the net effects of the equal and opposite moments produced by the advancing side blades of the counter-rotating rotors. The ability to off-load the retreating blades from producing lift alleviates retreating blade stall—a primary cause of speed limitation on conventional rotary wing aircraft—thereby permitting much greater forward flight speeds to be achieved.
Another consequence of high-speed flight is that the tip Mach number encountered by the advancing blades of a high speed rotary-wing aircraft is significantly higher than for conventional rotary-wing aircraft, while the retreating blades on the counter-rotating rotor operate in significant regions of reversed flow. Typically, conventional rotary-wing aircraft are limited to advance ratios of 0.4 to 0.45, encounter advancing side blade tip Mach numbers within 0.80 to 0.85, and typically have no more than 45% of the retreating blades immersed in reverse flow in high speed flight. High speed compound rotary wing aircraft are designed to attain advance ratios approaching 1.0, and encounter advancing blade tip Mach numbers greater than 0.9. Without rotor RPM scheduling, in which rotor tip speed is reduced with increasing flight velocity, the advancing side rotor blade tips may exceed sonic velocities which may be accompanied by significant compressibility drag and blade vibratory loads. Moreover, significantly larger portions of the retreating blades are immersed in reversed flow at high speeds; typically as much as 80% at advance ratios of 0.8. Furthermore, blade loadings at high speeds, even under normal operating conditions, are significantly higher than for conventional rotary wing aircraft.
In order to preserve the helicopter attributes of a high speed rotary wing aircraft, it is important to optimize its hover performance. The hover Figure of Merit of the above described dual, counter-rotating, coaxial rotor system developed to date is approximately 0.78. This is not considered to be particularly impressive hover performance. However, the primary performance parameter for the rotor blade is concentrated on forward flight speed capabilities. For this reason, and also because sophisticated hover optimization analyses have heretofore been unavailable, high-speed coaxial rotor aircraft hover performance has heretofore been acceptable at the predicted level.
Accordingly, it is desirable to provide a rotor blade for a high speed rotary-wing aircraft flight envelope that includes forward flight at speeds in excess of 250 kts with improved hover performance to maintain efficient helicopter type attributes and slow flight capabilities.